lanthanide amidinates and guanidinates: from laboratory ... · metallic chemistry of the lanthanide...

Lanthanide amidinates and guanidinates: from laboratory curiosities to

efficient homogeneous catalysts and precursors for rare-earth oxide thin

filmsw

Frank T. Edelmann

Received 9th October 2008

First published as an Advance Article on the web 8th April 2009

DOI: 10.1039/b800100f

For decades, the organometallic chemistry of the rare earth elements was largely dominated by the

cyclopentadienyl ligand and its ring-substituted derivatives. A hot topic in current organolanthanide chemistry

is the search for alternative ligand sets which are able to satisfy the coordination requirements of the large

lanthanide cations. Among the most successful approaches in this field is the use of amidinate ligands of the

general type [RC(NR0)2]� (R = H, alkyl, aryl; R0 = alkyl, cycloalkyl, aryl, SiMe3) which can be regarded as

steric cyclopentadienyl equivalents. Closely related are the guanidinate anions of the general type [R2NC(NR 0)2]�

(R = alkyl, SiMe3; R0 = alkyl, cycloalkyl, aryl, SiMe3). Two amidinate or guanidinate ligands can coordinate to

a lanthanide ion to form a metallocene-like coordination environment which allows the isolation and

characterization of stable though very reactive amide, alkyl, and hydride species. Mono- and trisubstituted

lanthanide amidinate and guanidinate complexes are also readily available. Various rare earth amidinates and

guanidinates have turned out to be very efficient homogeneous catalysts e.g. for ring-opening polymerization

reactions. Moreover, certain alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) were found to be

highly volatile and could thus be promising precursors for ALD (= Atomic Layer Deposition) and MOCVD

(= Metal-Organic Chemical Vapor Deposition) processes in materials science and nanotechnology.

This tutorial review covers the success story of lanthanide amidinates and guanidinates and their transition from

mere laboratory curiosities to efficient homogeneous catalysts as well as ALD and MOCVD precursors.

1. Principles of organolanthanide chemistry

The very beginning of organolanthanide chemistry dates back to

the year 1954, when Birmingham and Wilkinson reported

the synthesis of the first tris(cyclopentadienyl) lanthanide

complexes, Cp3Ln.1 However, a major drawback encountered

in this young field of organometallic chemistry was the intrinsic

instability of certain classes of organolanthanide complexes

combined with their often extreme sensitivity towards traces of

air and moisture. Thus until about 30 years ago organo-

metallic compounds of the rare earth metals remained a

curiosity. This situation changed slowly in the late 1970s and

the early 1980s when more sophisticated preparative and

analytical techniques became more generally available.2

Especially the use of dry-boxes and access to single crystal

X-ray diffraction techniques made it possible to safely handle

and characterize these compounds and to understand the struc-

tural features of these fascinating complexes. The area developed

even faster in the late 1980s, largely stimulated by the discovery

of the high potential of these compounds as reagents in organic

synthesis and as very active homogeneous catalysts.3

It should be emphasized at this point that the organo-

metallic chemistry of the lanthanide (and actinide) elements

differs significantly in several ways from that of the

d-transition metals.4,5 Generally accepted principles of organo-

d-transition metal chemistry do not apply to organolanthanide

compounds, including the well-known ‘‘18-electron rule’’,

s-donor/p-acceptor metal ligand bonding, the formation of

stable carbonyls and olefin, alkyne, carbene or carbyne

Chemisches Institut, Otto-von-Guericke-Universitat Magdeburg,D-39106, Magdeburg, Germany. E-mail: [email protected];Fax: +49-391-67-12933; Tel: +49-391-67-18327w Dedicated to Professor John W. Gilje on the occasion of his 70thbirthday.

Frank T. Edelmann

Frank T. Edelmann was bornin Hamburg, Germany, in1954. He studied chemistry atthe University of Hamburg,where he obtained his PhD in1983 with Ulrich Behrens.After 2 years of postdoctoralresearch with Josef Takats(University of Alberta), JohnW. Gilje (University ofHawaii) and Tristram Chivers(University of Calgary) hefinished his habilitation at theUniversity of Gottingen in1991 in the group of HerbertW. Roesky. In 1995 he was

appointed Full Professor of Inorganic Chemistry at theOtto-von-Guericke-University in Magdeburg. His main researchinterests are in organolanthanide and -actinide chemistry, siliconchemistry, and fluorine chemistry. His work is documented inover 280 scientific papers, 2 books and several patents. In 2008he was awarded the ‘‘Terrae Rarae 2008 Award’’ for his‘‘eminent work in coordination chemistry of the rare earthelements’’.

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 2253–2268 | 2253

TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews

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http://dx.doi.org/10.1039/B800100F

complexes, as well as the formation of stable MQO, MQN or

MRNmultiple bonds. In fact, even direct metal–metal bonds, a

common phenomenon in d-transition metal chemistry, are

virtually non-existent in organo-f-element chemistry. This

is due to several intrinsic properties of the lanthanide ions

imparting unique electronic and steric features to rare earth

organometallics. The rare earth metals (= Ln) generally

comprise the lanthanides La–Lu as well as the group 3 metals

scandium and yttrium. The 14 elements of the lanthanide series

represent the largest subgroup in the Periodic Table. All

lanthanide metals are highly electropositive, comparable to the

alkali and alkaline earth metals, so that metal-to-ligand bonding

can be considered as predominantly ionic. Therefore, the spectro-

scopic and magnetic properties of lanthanide coordination

compounds are largely uninfluenced by the ligands. According

to the HSAB (= hard and soft acids and bases) concept,

lanthanide ions are considered as hard acids. This hard acid

character combined with the largely ionic bonding is the origin of

the pronounced oxophilicity of the lanthanide ions which can be

quantified in terms of the dissociation energy of LnO. This

oxophilic nature of the metal centers strongly influences the

reactivity of lanthanide compounds and leads to a preference

for hard oxygen- or nitrogen-based ligands while coordination of

softer ligands containing e.g. phosphorus or sulfur donor atoms

is generally disfavored. In all cases, Ln3+ is the most stable

oxidation number, especially in aqueous solution. The most

easily accessible exceptions from the uniform Ln(III) oxidation

state are tetravalent cerium as well as the reduced Ln2+ state for

samarium, europium and ytterbium. When crossing the series

from La to Lu, a steady decrease in the Ln3+ ionic radii is

observed. This phenomenon is well known as the lanthanide

contraction which is basically caused by shielding of the

4f electrons by the 5s2 and 5p6 orbitals.4,5 The lanthanide

contraction causes the lanthanide ions to have similar, but not

identical, properties and is the main reason why separation of the

lanthanides can be achieved.

Lanthanide amidinates and guanidinates comprise a relatively

new class of rare earth metal complexes containing N-chelating

ligands. Such compounds have been known for less than 20 years.

When first reported around 1990, no practical uses were

envisaged for lanthanide amidinates. Since then, amidinates

(and the closely related guanidinates) of the rare earth

elements have turned out to be highly promising with respect to

applications in homogeneous catalysis andmaterials science alike.

This tutorial review tries to cover the whole success story of

lanthanide amidinates and guanidinates and their transition from

mere laboratory curiosities to efficient homogeneous catalysts and

ALD and MOCVD precursors. References in this article are

restricted mainly to publications illustrating the historic develop-

ment of the field as well as review articles. A recently published

review article entitled ‘‘Advances in the Coordination Chemistry

of Amidinate and Guanidinate Ligands’’ provides rapid access to

all original literature covered in this tutorial review.6

2. Amidinate anions: highly versatile

cyclopentadienyl-alternatives

Following the early discovery of the tris(cyclopentadienyl) com-

plexes of the lanthanide elements byWilkinson and Birmingham,1

most of the organolanthanide compounds studied were

sandwich-type complexes containing unsubstituted or ring-

substituted cyclopentadienyl ligands.3 Their relatively high

stability against moisture and air motivated numerous

research groups to develop the area of lanthanocene chemistry

over the last three decades. A main drawback encountered

with using unsubstituted Cp was certainly the difficulty

in obtaining anything other than Cp3Ln, the chemistry

of which is rather limited. With the introduction of Cp*

(= pentamethylcyclopentadienyl) ligands into organolanthanide

chemistry, more versatile compounds of the Cp*2LnX

type became more easily available and this opening of the

coordination sphere led to the development of a much

more diverse derivative chemistry. Subsequently, it was

demonstrated that various lanthanocene complexes exhibit

highly efficient catalytic activity for olefin transformations

including hydrogenation, polymerization, hydroboration,

hydrosilylation, hydroamination, and hydrophosphination.7,8

However, in this chemistry the steric saturation of the coordi-

nation sphere around the large f-element ions is much more

important than the number of valence electrons. Thus

current research in this area is increasingly focussed on the

development of alternative ligand sets. Amidinate and

guanidinate anions play a central role in this area of research.

An early account of lanthanide amidinate chemistry can be

found in a 1995 review article entitled ‘‘Cyclopentadienyl-free

Organolanthanide Chemistry’’.9–12

Amidinate anions of the general formula [RC(NR0)2]�

(Scheme 1) are the nitrogen analogues of the carboxylates.

They have been widely employed as spectator ligands in main

group and transition metal coordination chemistry, with the

latter encompassing both early and late transition metals as

well as the lanthanides and actinides. As illustrated in

Scheme 1, all three substituents at the heteroallylic N–C–N

unit can be varied in order to meet a large range of steric

(and, to a lesser degree, electronic) requirements.

In addition to the substituents listed in Scheme 1, chiral

groups may be introduced, and unsymmetrically substituted

amidinate anions are also possible. The amidinate anions may

also contain additional functional groups, or two such anions

can be linked with or without a suitable spacer unit. Yet

another variety comprises the amidinate unit being part of an

organic ring system.6

Historically, the amidinate story begins with the discovery

of N,N,N0-tris(trimethylsilyl)benzamidine, PhC(QNSiMe3)-

[N(SiMe3)2], by Sanger and co-workers.13 The compound

was prepared by the reaction of benzonitrile with

LiN(SiMe3)2 followed by treatment with chlorotrimethylsilane

(Scheme 2).

Scheme 1

2254 | Chem. Soc. Rev., 2009, 38, 2253–2268 This journal is �c The Royal Society of Chemistry 2009

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The method was later improved by Oakley et al.14 In their

1987 paper these authors also reported a series of p-substituted

derivatives. In the following years these N-silylated ligands

were extensively employed in main group and transition metal

chemistry, and the first review article covering the field was

published by Dehnicke in 1990.15 Since 1994 an incredible

variety of differently substituted amidinate ligands has been

developed and employed in the coordination chemistry of

various elements throughout the Periodic Table.16–18 The main

advantages of these ligands are twofold. Amidinate anions are

generally readily accessible using commercially available or

easily prepared starting materials. Furthermore, their steric

and electronic properties can be readily modified in a wide

range through variation of the substituents on the carbon

and nitrogen atoms. These properties combined make the

amidinate anions clearly almost as versatile as the ubiquitous

cyclopentadienyl ligands.

Metal amidinato complexes are principally accessible

through several synthetic routes. The most prevalent of them

include:

(i) insertion of carbodiimides, R–NQCQN–R, into a

metal–carbon bond;

(ii) deprotonation of an amidine using a metal alkyl;

(iii) salt metathesis reactions between a metal halide

substrate and an alkali metal amidinate (with the latter

normally being generated by one of the routes (i) or (ii));

(iv) reaction of metal halides with N,N,N0-tris(trimethylsilyl)-

amidines.

It should be noted here that not all these synthetic routes are

equally well applicable to the rare earth elements. Route (ii) is

severely restricted by the paucity of simple lanthanide

alkyls, while route (iv) is unsuitable in the lanthanide case as

lanthanide trihalides are generally unreactive towards N,N,N0-

tris(trimethylsilyl)amidines. Deprotonation of amidines by

metal amides should also be a possible synthetic pathway

since the delocalized structure of the resulting anion will

increase the acidity. However, this route has apparently not

yet been tried.

Closely related to the amidinate anions are the guanidinate

ligands (Scheme 3), which differ only in that they contain a

tertiary amino group at the central carbon atom of the NCN

unit. The beginning of their coordination chemistry dates back

to the year 1970, when Lappert et al. reported the first

transition metal guanidinate complexes.19 Like the amidinates,

these anions too make attractive ligands because of the

similar steric and electronic tunability through systematic

variations of the substituents at the carbon and nitrogen

atoms. The general synthetic methods for preparing metal

amidinato complexes can in part be applied to the corres-

ponding guanidinates and include the following:

(i) insertion of carbodiimides into a metal–nitrogen bond;

(ii) deprotonation of a guanidine using a metal alkyl;

(iii) salt metathesis reactions between a metal halide

substrate and an alkali metal guanidinate (with the latter

normally being generated by one of the routes (i) or (ii)).

Here too the same restrictions apply for the preparation of

lanthanide guanidinates. This means that route (ii) especially is

not generally applicable due to the very limited access to

simple lanthanide alkyls.

The general coordination modes of amidinate and guanidinate

(R = NR02) ligands are shown in Scheme 4. Both ligands

display a rich coordination chemistry in which both chelating

and bridging coordination modes can be achieved. By far the

most common coordination mode is the chelating type A. In

contrast, there are only rare examples of monodentate metal

coordination (B). This type of bonding can be the result of

severe steric crowding in certain amidinate or guanidinate

ligands containing very bulky substituents. Also very common

in transition metal chemistry is the bridging coordination

mode C. The bridging coordination mode is often found in

dinuclear transition metal complexes with short metal–metal

distances, i.e. multiple bonding between the metal atoms).

The majority of these complexes belong to the class of

‘‘paddlewheel’’-type compounds with the general formula

M2(amidinate)4 or M2(guanidinate)4. The fascinating chemistry

of these complexes has been extensively investigated, mainly

by Cotton and Murillo et al.6 However, this coordination

mode, which is so characteristic for various d-transition

metals, is completely unknown in lanthanide chemistry. The

synthesis of dinuclear complexes comprising any type of direct

Ln–Ln bonding (either single or multiple bonding) remains

one of the big challenges in organolanthanide chemistry today.

Even well-defined complexes containing an unsupported

Ln–M bond (M = d-transition metal) are extremely rare.20

The factors governing the formation of either chelating or

bridging coordination modes in amidinate and guanidinate

complexes have been analyzed in detail. Perhaps the greatest

advantage of these ligands besides their easy availability is the

possibility of tuning their steric requirements in a wide range

by variation of the substituents at carbon and nitrogen. To

Scheme 2

Scheme 3

Scheme 4


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some extent, the electronic properties can also be influenced

by introduction of suitable substituents such as SiMe3 or

2,4,6-C6H2(CF3)3.17 Amidinate and guanidinate ligands have

small N–M–N bite angles which are typically in the range of

63–651. Tuning of the steric demand of the ligand by varying

the substituents at nitrogen can also influence the coordination

geometry of the metal center in bis(amidinato) complexes.

Steric hindrance imparted by substituents on the nitrogen

atoms has its main effects mainly within the heteroallylic

N–M–N plane. Terphenyl substituents on the amidinate

carbon atom have been successfully employed to effect steric

shielding above and below the N–M–N plane, although

this shielding is fairly remote from the metal center.

Another approach to provide steric protection of the

N–M–N coordination plane is the use of ortho-disubstituted

aryl substituents on the nitrogen atoms. Especially useful in

this respect is the 2,6-diisopropylphenyl group. Also a very

important point to be noted is that in N,N0-bis(trimethylsilyl)-

benzamidinates and related ligands the orientation of the

phenyl ring on the central carbon atom with respect to the

N–C–N unit is normally nearly perpendicular. This conformation

accounts for the fact that such amidinate anions are not ‘‘flat’’

ligands such as the isoelectronic carboxylate anions, but also

extend above and below the N–C–N plane. It was first pointed

out by us that these ligands can be regarded as ‘‘steric

cyclopentadienyl equivalents’’.21 The concept of ‘‘steric

cyclopentadienyl equivalents’’ was developed by Wolczanski

et al. in connection with a series of tri-t-butylmethoxide

(‘‘tritox’’) complexes.22 The sterically demanding tri-t-butyl-

methoxide ligand forms a steric cone about a metal, similar to

bis(trimethylsilyl)amide or cyclopentadienyl. With 1251 the

cone angle of tritox approaches that of cyclopentadienyl

(1361). Amidinate and guanidinate ligands, too, are

monoanionic ligands, and their steric demand is tunable in a

wide range similar to the transition from unsubstituted

cyclopentadienyl to very bulky ring-substituted cyclopentadienyl

ligands.

3. Synthesis and characterization of lanthanide

amidinates and guanidinates

3.1 Lanthanide(II) amidinates and guanidinates

In (organo)lanthanide chemistry the divalent oxidation state

is most readily accessible under normal conditions for

samarium, europium, and ytterbium. Compounds of these

elements in the +2 oxidation state are stronger reducing

agents and undergo a large variety of interesting redox

reactions.3 Just as in cyclopentadienyl–lanthanide chemistry,

stable divalent species were of particular interest in amidinate

and guanidinate lanthanide chemistry due to their anticipated

high reactivity. In fact, soluble and very reactive ytterbium(II)

benzamidinates containing [RC6H4C(NSiMe3)2]� ligands were

reported as early as 1990.17 As shown in Scheme 5, these

compounds were readily prepared by treating freshly prepared

YbI2 in THF solution with two equivalents of sodium

N,N0-bis(trimethylsilyl)amidinates and isolated as dark red,

highly air-sensitive materials. X-Ray diffraction studies

revealed the trans-coordination with two chelating amidinate

ligands. A notable structural feature, characteristic also for

many other complexes containing these ligands, is the large

dihedral angle (77.31 for R =H) between the phenyl rings and

the amidinate N–C–N units which precludes any conjugation

between the two p-systems and is responsible for the cone-like

overall shape of the amidinate ligands.17

As expected, the ytterbium(II) benzamidinates shown in

Scheme 5 were found to be strong reducing agents and to

undergo various redox reactions e.g. with alkyl halides

and dichalcogenides REER (E = S, Se, Te). For example,

the S–S bond in [Me2NC(S)S]2 is readily cleaved to form an

ytterbium(III) dithiocarbamate complex (Scheme 6).17

Facile cleavage of E–E bonds (E = Se, Te) was also

observed (Scheme 7) giving rise to stable ytterbium(III)

amidinate complexes comprising Yb–Se and Yb–Te bonds.17

Scheme 5

Scheme 6

Scheme 7


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While ytterbium(II) benzamidinate complexes have been

known for many years, the synthesis of the first divalent

samarium bis(amidinate) required the use of a sterically hindered

amidinate ligand, [HC(NDipp)2]� (Dipp = C6H3Pr2

i-2,6). The

dark green samarium(II) bis(amidinate) Sm(DippForm)2(THF)2(DippForm = [HC(NDipp)2]

�) can be prepared by three

different synthetic routes (Scheme 8). The trivalent diiodo

samarate complex [Na(THF)5][SmI2(DippForm)2(THF)2]

was isolated in small quantities as a colorless byproduct.

Dissolution of this compound in hexane led to ligand

redistribution to give Sm(DippForm)3 with concomitant

precipitation of NaI and SmI3(THF)3.5. The monofluoro-

bis(amidinate) SmF(DippForm)2(THF) could also be isolated

from Sm(DippForm)2(THF)2 via treatment with Hg(C6F5)2.

These results clearly show that carefully selected amidinate

ligands are capable of stabilizing even the most strongly

reducing ‘‘classical’’ lanthanide(II) ion, i.e. divalent samarium.

Future investigations will show if similar complexes can also

be synthesized with ‘‘non-classical’’ divalent lanthanide ions

such as Tm2+, Nd2+ or Dy2+. Apparently, analogous

europium(II) amidinates too have not yet been reported,

although they should be readily accessible through standard

metathetical routes.6

In 2007 the first homoleptic lanthanide(II) guanidinate

complexes, Ln(Giso)2 (Ln = Sm, Eu, Yb; Giso =

Cy2NC(NAr)2; Ar = C6H3Pr2i-2,6), were reported and

shown to have differing coordination geometries (including

unprecedented examples of planar 4-coordination).23 In

particular, X-ray studies revealed planar 4-coordinate

coordination geometries for Sm and Eu derivatives (Fig. 1),

while the ytterbium(II) species is distorted tetrahedral.

In the case of Yb, two iodo-bridged dimeric lanthanide(II)

guanidinate complexes were also isolated from the reaction

mixtures (Fig. 2). The reaction of equimolar amounts of

Na[(Me3Si)2NC(NCy)2] (which was prepared in situ from

NaN(SiMe3)2 and N,N0-dicyclohexylcarbodiimide) and

YbI2(THF)2 gave the closely related ytterbium(II) guanidinate

complex [{(Me3Si)2NC(NCy)2}Yb(m-I)(THF)2]2 with bridging

iodo ligands.6,23

3.2 Lanthanide(III) amidinates and guanidinates

3.2.1 Homoleptic lanthanide(III) tris(amidinates) and

-guanidinates. Homoleptic lanthanide(III) tris(amidinates) and

-guanidinates are among the longest-known lanthanide

complexes containing these chelating ligands. The first

lanthanide amidinate complexes ever reported in the

literature were homoleptic tris(amidinates) of the type

[RC6H4C(NSiMe3)2]3Ln (Scheme 9), which can be regarded

as the amidinate analogues of the homoleptic lanthanide

tris(cyclopentadienyl) complexes Cp3Ln.6,21

A large number of such complexes were synthesized and

fully characterized in the course of the early studies. However,

at that time these compounds were mere laboratory curiosities

without any apparent practical use. It also turned out that the

derivative chemistry of these complexes is rather limited,

resembling that of the parent Cp3Ln complexes. It took many

years until the promising catalytic activities of these compounds

were discovered (see section 4).6,17

In a similar manner, treatment of anhydrous rare earth

chlorides with three equivalents of lithium 1,3-di-tert-

butylacetamidinate (prepared in situ from di-tert-butyl-

carbodiimide, tBu–NQCQN–tBu, and methyllithium) afforded

Scheme 8

Fig. 1 Molecular structure of Sm(Giso)2 (Giso = Cy2NC(NAr)2;

Ar = C6H3Pr2i-2,6).

Fig. 2 Molecular structure of [(Giso)Yb(THF)(m-I)]2 (Giso =

Cy2NC(NAr)2; Ar = C6H3Pr2i-2,6).

Scheme 9


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Ln[MeC(NBut)2]3 (Ln = Y, La, Ce, Nd, Eu, Er, Lu)

in 57–72% isolated yields.6,24 X-Ray crystal structures of

these complexes demonstrated monomeric formulations

with distorted octahedral geometry about the lanthanide(III)

ions (Fig. 3, Ln = La). The new complexes are thermally

stable at 4300 1C, and sublime without decomposition

between 180–220 1C/0.05 Torr. Other series of homoleptic

lanthanide tris(amidinates) include the N-cyclohexyl-

substituted derivatives [RC(NCy)2]3Ln(THF)n (R = Me,

Ln = Nd, Gd, Yb, n = 0; R = Ph, Ln = Nd, Y, Yb, n = 2).

A sterically hindered homoleptic samarium(III) tris(amidinate),

Sm[HC(NC6H3Pr2i-2,6)2]3, was obtained by oxidation of the

corresponding Sm(II) precursor (cf. Scheme 8).6

In this area the carbodiimide insertion route is usually

not applicable, as simple well-defined lanthanide tris(alkyls)

and tris(dialkylamides) are not readily available.3–5 A

notable exception is the formation of homoleptic lanthanide

guanidinates from anionic lanthanide amido complexes and

carbodiimides (Scheme 10).6

The major synthetic route leading to homoleptic lanthanide

tris(amidinates) and -guanidinates, however, is the metathetical

reaction between anhydrous lanthanide trichlorides and

preformed lithium amidinates or guanidinates, respectively,

in a molar ratio of 1 : 3 (Scheme 11).6

The six-coordinate tris(benzamidinates) [PhC(NSiMe3)2]3Ln

form 1 : 1 adducts with donor ligands such as THF, MeCN, or

PhCN. In the case of benzonitrile it was possible to isolate

and structurally characterize two of these seven-coordinate

nitrile-adducts. They are most readily isolated when the

original preparation is directly carried out in the presence of

one equivalent of benzonitrile (Scheme 12). The facile adduct

formation with small donor molecules is yet another analogy

between the tris(amidinates) and the tris(cyclopentadienyl)

complexes.6,17

In recent years homoleptic lanthanide(III) tris(amidinates)

and -guanidinates have been demonstrated to exhibit extremely

high activity for the ring-opening polymerization of polar

monomers such as e-caprolactone and trimethylene carbonate

(cf. section 4.1). Another promising field of application is their

use in CVD and ALD processes (cf. section 5).6

3.2.2 Lanthanide(III) bis(amidinates) and -guanidinates:

metallocene analogues. Bulky heteroallylic ligands such as the

benzamidinate anions [RC6H4C(NSiMe3)2]� soon became

well established as useful alternatives to the cyclopentadienyl

ligands. Shortly after the successful synthesis and character-

ization of various lanthanide(III) tris(amidinate) derivatives

the first disubstituted lanthanide(III) amidinate species were

reported. Such compounds appeared to be interesting

synthetic targets due to their metallocene-like coordination

environment which was expected to allow the isolation and

characterization of stable though reactive amide, alkyl,

and hydride species. In this respect the bulky amidinate and

guanidinate ligands resemble more the substituted cyclopenta-

dienyl ligands such as Cp*. One of the first representatives

of this class of compounds was a sky-blue neodymium

complex containing the very bulky nonafluoromesityl-

substituted benzamidinate ligand [2,4,6-(CF3)3C6H2C(NSiMe3)2]�

(Scheme 13).17

Retention of LiCl led to formation of an ‘‘ate’’

complex which is an amidinate analogue of the well known

neodymium metallocene complex Cp*2Nd(m-Cl)2Li(THF)2(Cp* = Z5-pentamethylcyclopentadienyl).3 While in the

case of the large Nd3+ ion the use of a sterically bulky

amidinate ligand was required, it soon turned out that

Fig. 3 Molecular structure of [MeC(NBut)2]3La.

Scheme 10

Scheme 11

Scheme 12

Scheme 13


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bis(benzamidinate) lanthanide chlorides could be accessed in a

more straightforward manner using scandium and yttrium

as smaller central ions.6,17 Teuben et al. were able to show

that the selective formation of lanthanide bis(amidinates)

could also be achieved by adjusting the steric requirements

of the ligands to the ionic radius of the lanthanide.

Metathetical reactions between ScCl3(THF)3 or YCl3(THF)3.5with two equivalents of the silyl-substituted amidinate

ligands afforded the corresponding bis(amidinato) complexes

[RC6H4C(NSiMe3)2]2LnCl(THF).25,26 A similar approach

using yttrium triflate as starting material was used to prepare

the first bis(amidinato) lanthanide triflate complexes

(Scheme 14).6,17

Following the successful isolation of these chloro and

triflate precursors, a major synthetic goal was the preparation

of reactive alkyl, amide, and hydride species based upon the

lanthanide bis(amidinate) moiety. This would further establish

a close analogy between lanthanide bis(amidinates) and

Cp*2Ln-type lanthanide metallocenes. For example, the

yttrium(III) benzamidinates [RC6H4C(NSiMe3)2]2YCH(SiMe3)2are analogues of the metallocene alkyls Cp*2YCH(SiMe3)2.

Indeed, it proved possible to employ the same synthetic

routes which were well established in lanthanide metallocene

chemistry to prepare the corresponding amidinate complexes.

For example, alkyl derivatives were found to be easily

accessible by alkylation of the corresponding chloride or

triflate precursors (Scheme 15). Following their successful

syntheses, it was demonstrated that these compounds exhibit

comparable catalytic activities e.g. in olefin polymerizations as

their metallocene counterparts (cf. section 5).6,10,17

Other s-alkyl yttrium complexes of this type include

[PhC(NSiMe3)2]2YCH2Ph(THF) and [PhC(NSiMe3)2]2Y-

(m-Me)2Li(TMEDA). The p-methoxy-substituted derivative

[MeOC6H4C(NSiMe3)2]2YCH(SiMe3)2 was characterized

by an X-ray structure determination confirming that the

bis[N,N0-bis(trimethylsilyl)benzamidinate] ligand system sterically

most resembles the bulky bis(pentamethylcyclopentadienyl)

rather than the bis(cyclopentadienyl) ligand set. The reactivity

of the benzamidinate-stabilized yttrium complexes towards

various reagents including hydrogen, ethylene, terminal

alkynes, nitriles, and pyridine derivatives was investigated

in detail. Hydrogenolysis of the yttrium alkyls in benzene

under 4 bar of H2 afforded the dimeric hydride

[{RC6H4C(NSiMe3)2}2Y(m-H)]2. Both [PhC(NSiMe3)2]2-

YCH2Ph(THF) and [{RC6H4C(NSiMe3)2}2Y(m-H)]2 were

modestly active in ethylene polymerization but did not react

with propylene and 1-hexene. The alkyls and the hydride were

found to react with a-picoline to give an a-picolyl derivativevia C–H activation of a methyl group (Scheme 16).6,17

Closely related benzamidinate chemistry has also been

investigated with scandium. Due to its small ionic radius,

Sc3+ forms disubstituted benzamidinate complexes which give

rise to interesting derivative chemistry. The main starting

material is [PhC(NSiMe3)2]2ScCl(THF), which is available

from the reaction of Na[PhC(NSiMe3)2] with ScCl3(THF)3in THF. The preparation of alkyl and aryl derivatives is

illustrated in Scheme 17 (Mes = mesityl).6,10,17

In the case of the methyllithium reaction the product is a

THF solvate while, with bulky hydrocarbyls, unsolvated

complexes were obtained. The compound [PhC(NSiMe3)2]2-

ScCH2SiMe3 readily reacts with H2 to form the dimeric

hydride [{PhC(NSiMe3)2}2Sc(m-H)]2.

Scheme 14

Scheme 15

Scheme 16

Scheme 17

Scheme 18


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Metathetical routes using bulky lithium guanidinates as

starting materials have also been employed to synthesize

bis(guanidinato) lanthanide halides (Scheme 18) as well as

reactive alkyls and hydrides.6

Treatment of [(Me3Si)2NC(NPri)2]2Ln(m-Cl)2Li(THF)2 with

methyllithium in the presence of TMEDA afforded the methyl

bridged ‘‘ate’’ complexes [(Me3Si)2NC(NPri)2]2Ln(m-Me)2-

Li(THF)2 (Ln = Nd, Yb) which also have analogues in

Cp*2Ln chemistry. With Ln = Y and Nd the unsolvated,

chloro-bridged dimers [{(Me3Si)2NC(NPri)2}2Ln(m-Cl)]2 were

also synthesized. Amination of these complexes with

two equivalents of LiNPri2 gave the amido derivatives

[(Me3Si)2NC(NPri)2]2LnNPr2i. The alkyl lutetium derivative

[(Me3Si)2NC(NPri)2]2Lu(CH2SiMe3) was synthesized via a

metathesis reaction of the chloro precursor with LiCH2SiMe3in toluene (Scheme 19). Treatment of the alkyl with an

equimolar amount of PhSiH3 in hexane led to formation of

the first known dimeric lanthanide hydride in a bis(guanidinato)

coordination environment (Scheme 20).6

Mutual ligand arrangement in binuclear lanthanide

guanidinate complexes of types [(guanidinate)2Ln(m-Cl)]2,[(guanidinate)2Ln(m-H)]2, and (guanidinate)2Ln(m-Cl)2Li(THF)2was quantitatively analyzed based on the ligand solid state angle

approach. In complexes of the larger early lanthanides Nd, Sm,

and Gd the guanidinate ligands shield up to 87% of the metal,

and the bidentate ligands on opposite metal centers are in an

eclipsed arrangement. In complexes of lanthanides with smaller

ionic radii such as Y, Yb, and Lu the guanidinate ligands

shield over 88.3% of the metal surface, and their staggered

conformation is observed. The same ligand sets, i.e. the

guanidinate anions [(Me3Si)2NC(NR)2]� with R = iPr and Cy,

were utilized to synthesize novel guanidinate borohydride

derivatives of lanthanides. For example, the heterobimetallic

Sm bis(guanidinate) complex [(Me3Si)2NC(NPri)2]Sm(m-BH4)2Li-

(THF)2 was obtained by a reaction of Sm(BH4)3(THF)2 with

two equivalents of the corresponding lithium guanidinate.

Subsequent treatment of the bimetallic ‘‘ate’’ complex, in

which Sm and Li are linked by two bridging borohydride

ligands, with 1,2-dimethoxyethane (DME) afforded the

ionic complex [Li(DME)3][{(Me3Si)2NC(NPri)2}2Sm(BH4)2]

(Scheme 21). The heterobimetallic ‘‘ate’’ complexes

[(Me3Si)2NC(NCy)2]Ln(m-BH4)2Li(THF)2 (Ln = Nd, Sm, Yb)

were found to catalyze methyl methacrylate polymerization

(cf. section 4.1).6

Bulky formamidinate ligands were also successfully

employed to prepare lanthanide bis(amidinates). Reaction of

La metal with Hg(C6F5)2 and bulky N,N0-bis(2,6-diisopropyl-

phenyl)formamidine (HDippForm) in THF (Scheme 22)

afforded (DippForm)2LaF(THF) with a rare terminal La–F

bond. A novel functionalized formamidine, resulting from

ring-opening of THF, was formed as a by-product in this

reaction. More recently these studies have been extended to

other lanthanide elements such as Ce, Nd, Sm, Ho, Er, and

Yb in order to gain a better understanding of the steric

modulation of coordination number and reactivity in the

synthesis of lanthanide(III) formamidinates.6

3.2.3 Lanthanide(III) mono(amidinates) and -guanidinates.

The last group of lanthanide(III) amidinates and guanidinates

to be synthesized were mono-substituted derivatives. As

in cyclopentadienyl lanthanide chemistry, saturating the

coordination sphere of the large lanthanide ions using just

one chelating or cyclic ligand proved the most difficult task

and often requires careful ligand design. For example, a novel

amidinate ligand incorporating a bulky terphenyl group was

used to prepare low-coordinate yttrium mono(amidinate)

complexes (Scheme 23).6

Especially notable among the most recent developments in

lanthanide amidinate chemistry is the successful synthesis of

stable mono(amidinate) yttrium dialkyls. The synthetic strategy

involved the use of the sterically hindered amidinate ligand

[ButC(NPri)2]�. The first complex was prepared by a sequential

reaction of YCl3(THF)3.5 with Li[ButC(NPri)2] (1 equiv.) and two

equivalents of LiCH(SiMe3)2 followed by pentane extraction.

Large crystals of [ButC(NPri)2]Y[CH(SiMe3)2]2(m-Cl)Li(THF)3

Scheme 19

Scheme 20

Scheme 21

Scheme 22


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were isolated in 83% yield and structurally characterized by

X-ray methods. Coordination of LiCl to the Y center results in a

five-coordinated molecule. Such a retention of alkali metal

halides which are formed during the course of reactions is a

characteristic phenomenon in organolanthanide chemistry leading

to the formation of so-called ‘‘ate’’ complexes. Subsequent

reactions of neutral bis(alkyl) lanthanide benzamidinates with

[NMe2HPh][BPh4] resulted in the formation of thermally

robust ion pairs (Scheme 24).6

In a systematic study it was demonstrated that, using a

specially designed bulky benzamidinate ligand, it is possible to

prepare mono(amidinato) dialkyl complexes over the full size

range of the Group 3 and lanthanide metals, i.e. from Sc to La

(Fig. 4). The synthetic routes leading to the neutral and

cationic bis(alkyls) are summarized in Scheme 25.6

Mono(guanidinato) complexes of lanthanum and yttrium

were synthesized as illustrated in Scheme 26.6

The La compounds were made starting from

La[N(SiMe3)2]3 and dicyclohexylcarbodiimide, Cy–NQCQN–Cy. Both mono(guanidinate) derivatives are monomeric

in the solid state with a four-coordinate La center. In the

case of yttrium it was recently reported that even the less

bulky guanidinate ligand [(Me3Si)2NC(NPri)2]� stabilizes

mono(guanidinato) complexes. Closely relatedmono(guanidinato)

lanthanide borohydride complexes of the type

[(Me3Si)2NC(NCy)2]Ln(BH4)2(THF)2 (Ln = Nd, Sm, Gd,

Er, Yb) were prepared by reactions of the corresponding

borohydrides Ln(BH4)3(THF)3 with the sodium guanidinate

Na[(Me3Si)2NC(NCy)2]. Scheme 27 illustrates the synthetic

route leading to the Gd derivative which was crystallized as an

adduct with 1,2-dimethoxyethane (DME).6

Bis(phenolate) ligands are present in the lanthanide guani-

dinate complexes shown in Scheme 28, which were prepared

by insertion of diisopropylcarbodiimide into the Ln–N bonds

of appropriate neutral lanthanide amide precursors.6

Scheme 23

Scheme 24

Fig. 4 Molecular structures of [PhC(NAr)2]Ln(CH2SiMe3)(THF)n

cations (Ln = Sc, n = 2; Ln = Gd, n = 3, Ln = La, n = 4)

Scheme 25

Scheme 26


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Brightly colored mono(benzamidinato) cyclooctatetraenyl

lanthanide(III) complexes were prepared by treatment of either

[(C8H8)Ln(m-Cl)(THF)2] or [(C8H8)Ln(m-O3SCF3)(THF)2]2with Na[RC6H4C(NSiMe3)2] (R = H, OMe, CF3)

(Schemes 29 and 30). According to X-ray structural analyses

of [PhC(NSiMe3)2]Tm(C8H8)(THF) and [MeOC6H4C(NSiMe3)2]

Lu(C8H8)(THF) these compounds are monomeric in the solid

state.6,10

Insertion of carbodiimides into the Ln–C s-bond of

organolanthanide complexes was also reported to provide

straightforward access to organolanthanide amidinates

and guanidinates. The complexes Cp2Ln[BunC(NBut)2]

(Ln = Y, Gd, Er) were prepared this way from Cp2LnBun

and N,N0-di-tert-butylcarbodiimide. The analogous insertion

of carbodiimides into Ln–N bonds has been shown to be an

effective way of making lanthanide guanidinate complexes.

For example, insertion of N,N0-dicyclohexylcarbodiimide into

one of the Y–N bonds of Y[N(SiMe3)2]3 afforded the

monoguanidinate diamide derivative [(Me3Si)2NC(NCy)2]Y-

[N(SiMe3)2]2. Complexes of the type Cp2Ln[Pri2NC(NPri)2]

(Ln = Y, Gd, Dy, Yb) have been prepared analogously.6

3.2.4 Lanthanide(III) complexes containing functionalized

amidinate ligands

(a) Pendant-arm type amidinate ligands. In order to further

explore the amidinate/cyclopentadienyl analogy, various

amino-functionalized amidinate ligands have been synthesized

with the aim of mimicking the well-known pendant-arm

half-metallocene catalysts. For example, benzamidinate

ligands incorporating a pendant amine functional group

were prepared as shown in Scheme 31. Instead of using

Me3SiCl/NEt3 in the first step, the starting amine can also

be converted into the monosilylated derivative by successive

reaction with nBuLi followed by treatment with Me3SiCl.6

Reactions of Li[Me3SiNC(Ph)N(CH2)3NMe2] (= LiL) with

MCl3 in appropriate stoichiometry led to the dinuclear

[(L)2M(m-Cl)]2 (M = La, Ce) complexes. The potentially

tridentate ligands [Me3SiNC(Ph)N(CH2)nNMe2]� (Scheme 32,

n= 2: B, n= 3: C) have also been employed in the synthesis of

yttrium alkyl and benzyl complexes.6

While the dialkyl complexes [Me3SiNC(Ph)N(CH2)n-

NMe2]Y[CH(SiMe3)2]2 could be isolated salt-free, attempts

to prepare analogous benzyl or trimethylsilylmethyl complexes

with ligand B yielded the ate-complexes Li[Me3SiNC(Ph)-

N(CH2)2NMe2]YR2 (R = CH2Ph, CH2SiMe3) in which the

Li ion is encapsulated by two amidinate and two amine

nitrogens. The increased spacer length between the amidinate

Scheme 27

Scheme 28

Scheme 29

Scheme 30 Scheme 31


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and amine functionalities in ligand C prevented encapsulation

of the Li ion, but still produced a bis(amidinate) yttrium

benzyl complex, [Me3SiNC(Ph)N(CH2)3NMe2]2YCH2Ph. In

this compound only one of the two NMe2 functionalities is

coordinated to the metal center.6

(b) Bis(amidinate) ligands. Yet another interesting variation

of the amidinate theme is the design and use of bis(amidinate)

ligands. This can for example involve the reductive dimerization

of carbodiimides to give oxalamidinate ligands of the

type [C2N4R4]2�. In the case of lanthanides, the reductive

coupling of carbodiimides was achieved with the use of

samarium(II) bis(trimethylsilyl)amides. The reaction of

Sm[N(SiMe3)2]2(THF)2 with carbodiimides RN = C = NR

(R = Cy, C6H3Pr2i-2,6) led to formation of dinuclear Sm(III)

complexes. For R = Cy (Scheme 33), the product has an

oxalamidinate [C2N4Cy4]2� ligand resulting from coupling of

two CyNQCQNCy moieties at the central C atoms. The same

product was obtained when the Sm(II) ‘‘ate’’ complex

Na[SmN(SiMe3)23] was used as divalent Sm precursor.6

The dilithium salt of a linked bis(amidinate) dianionic

ligand, Li2[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3] (= Li2L)

was prepared by reaction of dilithiated N,N0-bis(trimethylsi-

lyl)-1,3-diaminopropane with benzonitrile (Scheme 34).6

Reaction of the dilithium salt of the linked bis(amidinate)

dianionic ligand [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]2�

(= Li2L) with YCl3(THF)3.5 gave [Me3SiNC(Ph)N(CH2)3-

NC(Ph)NSiMe3]YCl(THF)2, which, by reaction with

LiCH(SiMe3)2, was converted to an alkyl complex

(Scheme 35). A structure determination of this compound

(Fig. 5) showed that linking together the amidinate

functionalities opens up the coordination sphere to allow

for the bonding of an additional molecule of THF not

present in the unbridged bis(amidinate) analogue

[PhC(NSiMe3)2]2Y[CH(SiMe3)2].6

Analogous reaction of Li2[Me3SiNC(Ph)N(CH2)3-

NC(Ph)NSiMe3] (= Li2L) with YbCl3 afforded the linked

Scheme 32

Scheme 33

Scheme 34

Scheme 35

Fig. 5 Molecular structure of [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]-

Y[CH(SiMe3)2](THF).

Scheme 36


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bis(amidinato) ytterbium chloride LYb(m-Cl)2YbL(THF). The

chloro-bridges in this compound are easily cleaved upon

treatment with THF. Further reaction of the resulting mono-

meric chloro complex with NaCp in DME gave LYbCp(DME)

in high yield (Scheme 36, Cp = Z5-cyclopentadienyl).6,27

In an extension of this work the steric effect of an amide

group on the synthesis, molecular structures and reactivity of

ytterbium amides supported by the linked bis(amidinate)

ligand was investigated. Reactions of the chloro precursor

with sodium arylamides afforded the corresponding mono-

metallic amide complexes in which the linked bis(amidinate) is

coordinated to the ytterbium center as a chelating ligand

(Scheme 37). In contrast, the reaction with NaN(SiMe3)2gave a bimetallic amide complex in which two linked

bis(amidinates) act as bridging ancillary ligands connecting

two YbN(SiMe3)2 fragments in one molecule.6,27

3.3 Lanthanide(IV) amidinates and guanidinates

Most recently it was discovered in our lab that benzamidinate

ligands are also capable of stabilizing novel cerium(IV) species.

Thus they belong to the limited number of organic ligands

which allow the stabilization of lanthanide complexes in all

possible oxidation states (+2, +3, and +4). In analogy to

the reaction shown in Scheme 9, treatment of anhydrous

cerium(III) trichloride with three equivalents of the lithium

amidinate precursor Li[p-MeOC6H4C(NSiNe3)2] first afforded

the bright yellow p-anisonitrile adduct of the homoleptic

cerium(III) amidinate (Scheme 38). Oxidation to the corres-

ponding cerium(IV) amidinate [p-MeOC6H4C(NSiMe3)2]3CeCl

was readily achieved using the reagent phenyliodine(III)

dichloride. The advantage of this method is that iodobenzene

is formed as the only by-product. This way the almost black

cerium(IV) species could be isolated by simple crystallization

directly from the concentrated reaction mixture. A single-

crystal X-ray analysis clearly established the presence of the

first amidinate complex of tetravalent cerium (Fig. 6).6

4. Lanthanide amidinates and guanidinates in

homogeneous catalysis

At the time when the first lanthanide amidinates were reported

in the early 1990s, these compounds appeared to be just

laboratory curiosities for which nobody could envisage any

practical uses. This situation changed completely when in 2002

it was discovered that homoleptic lanthanide tris(amidinate)

complexes show extremely high activity for the ring-opening

polymerization of e-caprolactone at room temperature.28 This

was the first time that promising catalytic activity was found

for lanthanide amidinates and it stimulated further work in the

field. Certain aspects of catalytic applications of lanthanide

amidinate complexes have already been summarized in review

articles.6,12

4.1 Polymerization reactions catalyzed by lanthanide

amidinate and guanidinate complexes

The use of amidinato catalyst systems for the polymerization

of olefins was first reported in a patent application by us in

cooperation with BASF.6 Since then various other patents

dealing with the use of amidinato metal complexes as catalysts

for olefin polymerizations have appeared. However, the main

focus of these studies was on the use of group 4 metal

amidinates as these have been found to exhibit very promising

activities as homogeneous polymerization catalysts. This area

has recently been compiled in an excellent review article by

Eisen and Smolensky.29 It took several more years until

lanthanide amidinates and guanidinates successfully entered

catalysis research and proved to be effective catalysts

especially for polymerization reactions. The majority of the

work published thus far involves the polymerization of

polar monomers such as e-caprolactone (CL), D,L-lactide or

Scheme 37

Scheme 38

Fig. 6 Molecular structure of [p-MeOC6H4C(NSiMe3)2]3CeCl.


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methylmethacrylate (MMA), but promising results have

also been achieved with simple olefins. For example,

the dimeric bis(guanidinato) lutetium hydride complex

[{(Me3Si)2NC(NPri)2}2Lu(m-H)]2 was found to catalyze the

polymerization of ethylene, propylene, and styrene. Moderate

activity in ethylene polymerization was observed with the

mono(guanidinato) yttrium dialkyl [(Me3Si)2NC(NPri)2]Y-

(CH2SiMe3)2(THF)2.6 The mono(amidinato) lanthanide

alkyls shown in Scheme 25 allowed a comparison of

the catalytic performance of cationic group 3 metal and

lanthanide alkyls in catalytic ethylene polymerization

over the full ion size range of these metals. Ethylene

polymerization experiments were performed using the metal

dialkyl precursors in combination with the Brønsted activator

[Ph2NMe2H][B(C6F5)4] and excess isobutyl alumoxane

(TIBAO) scavenger. It was found that the catalytic activity

of the intermediate cationic alkyl species varies by over 2

orders of magnitude with the size of the metal ion. Thus the

availability of such a large series of homologous precatalysts

allows a unique tunability of the catalytic activity.30

As mentioned above, the strength of lanthanide amidinate

and guanidinate catalysts lies in the polymerization of polar

monomers. In the quest for new single-site catalysts for

polymerizations of cyclic esters, a series of mononuclear

yttrium(III) complexes were synthesized, which are depicted

in Scheme 39.6

Coordination numbers ranging from 5 to 7 were observed,

and they appeared to be controlled by the steric bulk of the

supporting amidinate and coligands. Complexes 2–5 and 7

were found to be active catalysts for the polymerization of

D,L-lactide (e.g. with 2 and added benzyl alcohol, 1000 equiv.

of D,L-lactide were polymerized at room temperature in less

than 1 h). The neutral complexes 2, 5, and 7 were more

effective than the anionic complexes 3 and 4. Polymerization

of D,L-lactide to polylactide was also achieved using the

mono(guanidinate) lanthanum complex [(Me3Si)2NC(NCy)2]-

La[N(SiMe3)2]2. Although a high molecular weight polymer

was obtained, polydispersities were broad and no control over

the stereochemistry of the polymer was observed.6

The methyl bridged ‘‘ate’’ complexes [(Me3Si)2NC(NPri)2]2-

Ln(m-Me)2Li(THF)2 (Ln = Nd, Yb) exhibit extremely high

activity for the ring-opening polymerization of e-caprolactoneto give high molecular weight polymers. The Nd derivative

also showed good catalytic activity for the syndiotactic

polymerization of methylmethacrylate. Other metal

amidinates and guanidinates, for which high activity in the

polymerization of polar monomers such as e-caprolactone(CL), lactide, and methylmethacrylate (MMA) have been

reported, include the homoleptic lanthanide amidinates

Ln[RC(NCy)2]3(THF)n (R = Me, Ln = Nd, Gd, Yb,

n = 0; R = Ph, Ln = Y, Nd, Yb, n = 2)28 and the

bis(guanidinato) lanthanide amides [(Me3Si)2NC(NPri)2]2-

LnN(Pri)2 (Ln = Y, Nd). The neodymium compound

[MeC(NCy)2]3Nd was studied by Ephritikhine et al.

and compared to the catalytic activity of its uranium(III)

counterpart, [MeC(NCy)2]3U. The Nd complex showed again

extremely high activity in the ring-opening polymerization of

e-caprolactone (CL). In the presence of 0.5% mol equiv.

of [MeC(NCy)2]3Nd in toluene, polymerization of CL

was achieved in less than 2 min, giving a viscous gel of

poly(e-caprolactone). It was found that the uranium(III)

complex was much less efficient because of its rapid oxidation

into U(IV) species.31

Recently the guanidinate lanthanide borohydrides

[(Me3Si)2NC(NCy)2]Ln(BH4)2(THF)2 (Ln = Nd, Sm, Er, Yb)

and [(Me3Si)2NC(NCy)2]2Ln(BH4)2Li(THF)2 (Ln = Nd, Sm,

Yb) were reported to be promising monoinitiators for the

ring-opening polymerization of racemic lactide as well as

methyl methacrylate polymerization.6,32 Recently ytterbium

amide complexes stabilized by linked bis(amidinate) ligands

(cf. Scheme 35) were reported to be efficient initiators for the

polymerization of L-lactide. The catalytic performance was

found to be highly dependent on the amido groups and

molecular structures.6

The ring-opening polymerization of trimethylene carbonate

(TMC) using the homoleptic lanthanide amidinate complexes

[RC(NCy)2]3Ln (R = Me, Ph; Ln = La, Nd, Sm, Yb) as

single-component initiators was investigated in detail. It was

found that the substituents on the amidinate ligands and the

metal centers had a great effect on the catalytic activities of

these complexes, i.e. Me 4 Ph, and La 4 Nd 4 Sm 4 Yb.

Thus the lanthanum acetamidinate [MeC(NCy)2]3La showed

the highest catalytic activity. The ring-opening polymerization

of trimethylene carbonate was also achieved using homoleptic

lanthanide guanidinate complexes [R02NC(NR)2]3Ln. Here

too, the metal centers and the substituents on the guanidinate

ligands showed a marked effect on the catalytic activities withScheme 39


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[Ph2NC(NCy)2]3Yb being the most active catalyst in this case.

The copolymerization of TMC with e-caprolactone initiated by

[Ph2NC(NCy)2]3Yb was also tested. The other members of this

series of homoleptic lanthanide(III) guanidinates also exhibited

extremely high activity for the ring-opening polymerization of

e-caprolactone giving polymers with high molecular weights. In

this case, too, the different substituents at the guanidinate

ligands have a great effect on the catalytic activity.6

Although polymerization catalysis using lanthanide amidinates

and guanidinates is still in its infancy, the present results clearly

show the great potential this class of rare earth complexes offers.

Catalytic activities can be tuned in a wide range by using various

group 3 and lanthanide metals and different substituents at the

ligand backbone. A major advantage of the amidinates and

guanidinates over the established lanthanide metallocene catalysts

is their significantly improved air-stability especially in the case of

the tris(amidinates).

4.2 Other reactions catalyzed by lanthanide amidinate and

guanidinate complexes

An interesting organolanthanide-catalyzed reaction which

has been studied in recent years is the addition of terminal

alkynes to carbodiimides leading to the novel class of

N,N0-disubstituted propiolamidines. It was found that

half-sandwich rare earth metal complexes bearing silylene-

linked cyclopentadienyl-amido ligands can act as excellent

catalysts in this addition reaction. As illustrated in

Scheme 40, a rare earth amidinate species has been confirmed

to be a true catalytic species.

This organolanthanide-catalyzed reaction provides the first

example of efficient preparation of well defined propiolamidines,

a new family of amidines which are difficult to access by other

means and may show unique reactivity. A wide range of

terminal alkynes could be used for this catalytic cross-coupling

reaction which was not affected by either electron-withdrawing

or -donating substituents or their positions at the phenyl ring of

the aromatic alkyne.33

More recently it was demonstrated that this organolanthanide-

catalyzed reaction is more general in nature and can be extended

to other lanthanide precatalysts as well as the addition of N–H

bonds to carbodiimides.34,25 For example, the ring-bridged

metallocenes (EBI)LnN(SiMe3)2 (Ln = Y, Sm, Yb; EBI =

ethylene-bis(Z5-indenyl)) were found to exhibit versatile catalytic

activities with high efficiency toward the addition of the N–H

bonds of amines and the C–H bonds of terminal alkynes to

carbodiimides, thereby providing access to various substituted

guanidines and propiolamidines. Here again lanthanide

guanidinate and amidinate species are the key intermediates in

the proposed catalytic cycle.34 Various divalent lanthanide

complexes such as Ln[N(SiMe3)2]2(THF)3 (Ln = Sm, Eu, Yb)

or (MeC5H4)2Sm(THF)2 were also found to be highly active

precatalysts for the addition of N–H and C–H bonds to

carbodiimides. Scheme 41 illustrates the proposed catalytic cycle

for a samarium(II) silylamide precursor. The first step in both

reactions is supposed to be the formation of a bimetallic

samarium bis(amidinate) species originating from the reductive

coupling of carbodiimide promoted by the Ln(II) complex. The

active species is proposed to be a lanthanide guanidinate and a

lanthanide amidinate.35

The catalytic activity of bulky amidinato bis(alkyl)

complexes of scandium and yttrium (cf. section 3.2.3) in the

intramolecular hydroamination/cyclization of 2,2-dimethyl-4-

pentenylamine has also been investigated and compared to the

activity of the corresponding cationic mono(alkyl) derivatives.

Cyclotrimerization of aryl isocyanates leads to perhydro-1,3,5-

triazine-2,4,6-triones (isocyanurates). Another good catalyst

for the cyclotrimerization of phenyl isocyanate was found in

the organoyttrium amidinate complex Cp2Y[BunC(NBut)2].6

These initial results indicate that lanthanide amidinates and

guanidinates may exhibit interesting catalytic activities in

reactions other than polymerizations. Clearly more work in

this direction would be highly desirable.

5. Lanthanide amidinate and guanidinate

complexes in materials science

Even more surprising than the discovery of lanthanide

amidinate and guanidinate catalysis was the finding that such

complexes may turn out to be valuable precursors in materials

science and nanotechnology. Although this field of application

too is still in its infancy, the first results are promising and

indicate that volatile lanthanide amidinates could play a

significant role as ALD precursors in the future.Scheme 40

Scheme 41


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Atomic layer deposition (= ALD) is a modified chemical

vapor deposition (= CVD) process for the deposition of

highly uniform and conformal thin films by alternating

exposures of a surface to vapors of two chemical reactants.

Such uniform, conformal thin films have a wide variety

of applications in modern technology, including micro-

electronics. In 2003, Gordon et al. first reported that pure

metals can be deposited by using volatile homoleptic

N,N0-dialkylacetamidinato metal complexes and molecular

hydrogen gas as the reactants.36,37 A series of homoleptic

metal amidinates of the general type [M{RC(NR 0)2}n]x(R = Me, But; R = Pri, But) was prepared for numerous

metals (e.g. Ti, V, Mn, Fe, Co, Ni, Cu, and Ag), including

lanthanum (Scheme 42). The new compounds were found to

have properties well-suited for use as precursors for atomic

layer deposition of thin films. They have high volatility, high

thermal stability, and high and properly self-limited reactivity

with molecular hydrogen, depositing pure metals, or water

vapor, depositing metal oxides.36,37 Following this discovery,

several patents dealing with the use of volatile metal amidinate

complexes in MOCVD or ALD processes have meanwhile

appeared in the literature.6

ALD in the presence of water vapor has been successfully

employed for the deposition of lanthanide oxide thin

layers.36–39 There is considerable current interest in lanthanide

oxides such as La2O3 due to their potential applications as

insulators for memory and logic devices. Winter et al. first

reported the use of lanthanide amidinate complexes in the

fabrication of lanthanide-doped inorganic phases by CVD

methods. In the course of this work several new lanthanide

tris(amidinates) were prepared which sublime at temperatures

as low as 90 1C at low pressure. It was pointed out that

lanthanide amidinate complexes are extremely promising

CVD precursors due to their all-nitrogen coordination

spheres, relatively low molecular weights, and variability of

the amidinate ligand.38 Rare earth oxides, including Er2O3,

have high permittivities and are promising candidates to

replace SiO2 in future microelectronics devices. The atomic

layer deposition of Er2O3 films has been demonstrated using

[MeC(NBut)2]3Er and ozone with substrate temperatures

between 225–300 1C. The growth rate increased linearly with

substrate temperature from 0.37 A per cycle at 225 1C to

0.55 A per cycle at 300 1C (Fig. 7). The as-deposited films were

amorphous below 300 1C, but showed reflections due to cubic

Er2O3 at 300 1C.39

Most recently, eight novel homoleptic tris(guanidinato)

complexes M[(NPri)2CNR2]3 [M = Y, Gd, Dy; R = Me,

Et, Pri) have been synthesized and characterized by NMR,

CHN-analysis, mass spectrometry and IR spectroscopy. Single

crystal structure analyses revealed that all these compounds

are monomers with the rare-earth metal center coordinated to

six nitrogen atoms of the three chelating guanidinato ligands

in a distorted trigonal prism geometry. With the use of

TGA/DTA and isothermal TGA analysis, the thermal

characteristics of all tris(guanidinates) were studied in detail

to evaluate their suitability as precursors for thin film deposition

by MOCVD and ALD. The Pri-Me2N-guanidinates of Y, Gd

and Dy showed excellent thermal characteristics in terms of

thermal stability and volatility. Additionally, the thermal

stability of the Pri-Me2N-guanidinates of Y and Dy in solution

was investigated by carrying out NMR decomposition

experiments and both the compounds were found to be

remarkably stable. All these studies indicated that Pri-Me2N-

guanidinates of Y, Gd, and Dy have the prerequisites for

MOCVD and ALD applications which was confirmed by the

successful deposition of Gd2O3 and Dy2O3 thin films on

Si(100) substrates. The MOCVD grown films of Gd2O3 and

Dy2O3 were highly oriented in the cubic phase, while the ALD

grown films were amorphous.40

Yet another exciting application of the same erbium

amidinate precursor, [MeC(NBut)2]3Er, is its use as a dopant

source in the preparation of silicon nanocrystals. These nano-

crystals were prepared by the co-pyrolysis of [MeC(NBut)2]3Er

and disilane in a dilute helium stream at 1000 1C. Under

conditions identical to those used previously for b-diketonateprecursors, nanocrystals doped using this amidinate source

were larger in size, of a narrower size distribution, and

contained more erbium in the nanocrystal on average, making

them attractive for possible optoelectronic device structures.41

6. Conclusions

This tutorial review describes the success story of lanthanide

amidinates and guanidinates. First reported almost two

decades ago, these compounds have undergone a transition

from mere laboratory curiosities to efficient polymerization

catalysts and ALD precursors in recent years. Thus this article

provides a good example for purely academic research

eventually turning out to become the basis for unexpected

practical uses. Initial results clearly show that certain lanthanide

amidinates and guanidinates show very high catalytic activity

Scheme 42

Fig. 7 Growth rate of Er2O3 films deposited from [MeC(NtBu)2]3Er.


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in polymerization reactions of olefins and polar monomers

while other lanthanide amidinates are starting a career as

novel precursors in materials science and nanotechnology.

Besides these exciting applications, the synthetic and structural

chemistry of lanthanide amidinates is surprisingly diverse and

far from being fully exploited. Amidinate and guanidinate

ligands rival the ubiquitous cyclopentadienyl ligands in their

accessibility and versatility. Important aspects of lanthanide

cyclopentadienyl chemistry can now be realized using the

amidinate and guanidinate anions as ‘‘steric cyclopentadienyl

equivalents’’. Mono-, di- and trisubstitution products are

available as are complexes containing pendant-arm amidinates

and bis(amidinate) ligands. All three possible oxidation states

(+2, +3, and +4) of the lanthanide ions can be stabilized

with the use of amidinate ligands. Future preparative work

will show if e.g. novel divalent lanthanide amidinate and

guanidinate complexes (Nd2+, Eu2+, Dy2+, Tm2+) can be

prepared and fully characterized. Further investigation of the

recently discovered cerium(IV) benzamidinates is also clearly

warranted. Another point worthy of a more thorough

investigation would be if and when there is any marked

difference in catalytic activities between analogous cyclo-

pentadienyl and amidinate/guanidinate chemistry. It would

be interesting to see if there are examples of compounds

belonging to the same class of compounds (e.g. L2LnX) where

the reactivity is different or polymerization is better with

one class or other. This could be a good subject for more

systematic future studies. In any case it needs no prophet to

foresee that the most significant future development in this

area lies in the practical exploration of lanthanide amidinates

and guanidinates in catalysis, materials science and, perhaps,

as novel reagents in organic synthesis.

Acknowledgements

Financial support of the author’s own work on lanthanide

amidinates provided by the Deutsche Forschungsgemeinschaft

(SPP1166 ‘‘Lanthanoid-spezifische Funktionalitaten in

Molekul und Material’’), the Fonds der Chemischen Industrie,

and the Otto-von-Guericke-Universitat Magdeburg is most

gratefully acknowledged.

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